Fiber laser cleaning of metal mirror surfaces for optical diagnostic systems of the ITER
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r Laser Cleaning of Metal Mirror Surfaces for Optical Diagnostic Systems of the ITER A. P. Kuznetsova, A. S. Alexandrovaa, O. I. Buzhinskyb, K. L. Gubskiya, T. V. Kazievaa, A. V. Savchenkova, and S. N. Tugarinovb a
b
National Research Nuclear University MEPhI, Kashirskoe sh. 31, Moscow, 115409 Russia State Research Center of the Russian Federation Troitsk Institute for Innovation and Fusion Research, ul. Puskhovykh, vlad. 12, Troitsk, Moscow, 142092 Russia email: [email protected] Received October 21, 2014
Abstract—The results of experimental studies into efficiency of removal of films with a complex composition from metal mirrors by pulsed fiber laser irradiation are presented. It is shown that the initial reflectivity of optical elements can be restored by the selection of modes of irradiation impacting the surface with the sput tered film. Effective cleaning is performed by radiation with a power density lower than 107 W/cm2. The removal of contaminations at such a relatively low power density occurs in a solid phase, owing to which the thermal effect on the mirror is insignificant. Keywords: optical diagnostics of ITER, laser cleaning of mirror surfaces, fiber laser. DOI: 10.1134/S1063778815100063
INTRODUCTION A complex of optical diagnostics measuring the plasma characteristics is intended to be used at the ITER thermonuclear reactor. Optical elements of diagnostic systems arranged inside the vacuum cham ber of the reactor are subjected to intense radiation impact, sputtering by chargeexchange atoms, and contamination due to the repeated deposition of sput tered materials of the first wall, limiters, diverter plates, and other elements. The most vulnerable ele ment of optical systems is the first mirror, the working surface of which is directed to plasma in the reactor chamber. The first mirror should satisfy a series of requirements; notably, it should have high reflectance and high thermal conductivity, possess the necessary chemical stability, and be stable to transmutations; and its material should conserve its properties under strong neutron fluxes [1–3]. Currently, researchers consider Cu, W, Mo, stainless steel SS316, Al, Rh, V, Ag, Au, Ni, and Cr as the main materials for the fabri cation of the first mirror [4, 5]. Operating conditions and requirements on mirrors in the ITER for various types of diagnostics are substantially different. Cur rently, molybdenum is selected as the candidate mate rial of the first mirror for a series of optical diagnostics, particularly, charge exchange recombination spectros copy (CXRS) and Halpha spectroscopy (HAS). When performing simulation experiments on the investigation into the degradation and modification of mirror surfaces under conditions close to that
expected in the ITER, samples made of stainless steel SS316 can be used. The experimental investigation on the exposure of metal mirrors performed using tokamaks T10 [6], Tor Supra, TEXTOR [7], DIIID [8], and JET [9] showed that the largest variation in the mirror reflectance is caused by deposi
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